Hey Jay once you get everything you need to start working on the lower reciever. I would recommend buying plastic blocks first. They don't cost much and by using them your not waisting the expensive metal while you are working the bugs out

This seems like more than what's needed so I'm hoping somebody here can set me straight. It would be nice to have as little software as possible for simplicity's sake, so if there's a one stop software available on the market that would be great. If not, can somebody please recommend reliable, professional grade software for CNC milling?

I really do appreciate the responses.

Unfortunately its not more than what's needed. Each step of the process requires a different program. You need a CAD program to design it, a 3D modeling program to change your 2D drawing into a 3D piece (this step can be skipped but i wouldn't recommend it. I can't tell you how many projects have been saved because an oversight in the drawings became visible in the modeling), then you need a program to change your CAD drawings into a file where you can create tool paths, then you need a program to run the mill.
For the CAD program i STRONGLY suggest AutoCAD by Autodesk. Its easily the best CAD software out there. It has 3D modeling in it so that would eliminate a program for you. Disclaimer: there is a learning curve with it. It takes some time, practice, and perseverance to learn. But it IS the BEST CAD software on the market. Go on Ebay and get a student copy for $5 if you don't want to spend $2000+ on it. It will have a stamp on it that says "produced with an autodesk educational product"but other then that works just as the full version does.
Solidworks is a great modeling program. Easy to use and inexpensive.
As for the last two programs you need...I've been away from CNCing for a while but the manufacturer of your mill/lathe should be able to give you a good idea on that or may even give it to you with your product